1. Field of the Invention
The present invention relates to a speaker, and more particularly to a speaker including a substantially hemispherical surface vibrator.
2. Description of the Related Art
The inventor of the present invention has invented a speaker including a substantially hemispherical surface vibrator, which is described in Japanese Patent Application No. 7-347884 (Japanese Unexamined Patent Publication No. 9-168194). In this speaker 1, as shown in FIG. 7, electrodes 3a and 3b are provided on both surfaces of a substantially hemispherical surface vibrating body 2 made of a piezoelectric material. By inputting a signal between the electrodes 3a and 3b, the vibrating body 2 is vibrated so that an acoustic wave is radiated. The edge portion of the vibrator 2 is attached to a base 4 using an adhesive.
A horn portion 5 is provided in the base 4 having a hole (sound path) enlarging gradually from the inner surface side of the vibrating body 2 toward the outside. With the horn portion 5, an acoustic wave radiated from the inner surface side of the vibrating body 2, caused by the vibration of the vibrator 2, passing through the horn portion 5, is directed to the outside. An acoustic wave radiated from the outer surface side of the vibrating body 2 and an acoustic wave radiated from the inner surface side have a phase difference of 180xc2x0. However, the sound waves are manipulated by the horn portion 5 so as to have substantially the same phase in the direction parallel to the installation plane of the speaker 1.
In the above-described speaker, with an input signal, the entire substantially hemispherical surface vibrator expands or shrinks, that is, xe2x80x9cbreathing-vibratedxe2x80x9d, so that an acoustic wave is radiated. However, if the bonding strength between the vibrator and the base is weak, additional vibration other than the xe2x80x9cbreathing-vibrationxe2x80x9d is generated in the edge portion. When the breathing-vibration and the vibration in the edge portion are present, the sound pressure characteristic diminishes as a whole, and a sufficient sound pressure characteristic in the whole range of a required frequency band cannot be obtained. Further, thermal shock testing is carried out to confirm the reliability of a speaker. If the bonding strength of the adhesive is weak, the vibrator may peel away from the base, caused by the difference between the coefficients of thermal expansion of the vibrator and the base.
Moreover, if the bonding strength of the adhesive is too high, the vibration, generated in the edge portion of the vibrator, is suppressed. However, where the bonding strength is excessively high, the breathing-vibration of the vibrator is severely divided, and the base vibrates with the vibration leakage from the vibrator. For this reason, a distinct peak (a high portion where a sound level is prominent) and dip in sound pressure occur in the sound pressure characteristics. Thus, good sound pressure characteristics cannot be obtained. Further, in reliability testing, the vibrator may be broken, caused by the difference between the coefficients of thermal expansion of the vibrator and the base.
In order to overcome the problems described above, preferred embodiments of the present invention provide a speaker having high reliability, in which excellent sound pressure characteristics are obtained.
According to one preferred embodiment of the present invention, a speaker having a substantially hemispherical surface vibrator, a base bonded to the edge portion of the vibrator, a first adhesive disposed between the edge portion of the vibrator and the base to bond the vibrator to the base, and a second adhesive disposed at the circumference of the edge portion of the vibrator bonded by the first adhesive.
In this speaker, the vibrator is preferably a substantially hemispherical surface vibrating body including a piezoelectric material, and electrodes provided on the inner surface and the outer surface of the vibrating body.
The first adhesive is used to obtain an appropriate bonding strength between the vibrator and the base. The second adhesive is used to obtain a bonding strength between the vibrator and the base and to damp the vibration of the vibrator. The adhesives are selected such that the second adhesive has a lower elastic modulus than the first adhesive.
In the speaker according to one preferred embodiment of the present invention, a desired bonding strength is obtained, and moreover, vibration leakage from the vibrator is absorbed by the two types of adhesives.
The first adhesive is used to obtain an appropriate bonding strength between the vibrator and the base. Further, the second adhesive is used to obtain a bonding strength between the vibrator and the base, and moreover, functions as a damping material to prevent the vibration of the vibrator from leaking out.
To achieve the above-described effects, the second adhesive has a lower elastic modulus than the first adhesive.